1. open the sig that you want to use.
2. Make a new layer
3. Go Select > All, or press Ctrl + A
Now you should have a dotted line all around your sig like so:



4. Go Edit > Stroke, or press M(which selects your Marquee tool) and right click in your sig.



5. Use the stroke settings below:



Use black: #000000 then click Ok

6. Then open up the Stroke window again and use these settings:



Use white: #FFFFFF then click Ok
7. Your sig should look like this now:



Now its up to you to choose how you want to encorperate the border into your sig. You can use the layer blending modes to change how the border looks. some examples: